This invention relates to improvements in rotary gear pumps, more particularly to sealing means and arrangements in a rubber-geared pump to substantially isolate the inlet side from the outlet side thereof.
Theoretically, rotary positive displacement pumps, e.g., gear pumps, discharge a constant liquid volume per revolution. However, practically, they exhibit slip defined as leakage of fluid from the high pressure to the low pressure side through clearances between the meshed gears and between the rotating elements and the stationary pump housing. A gear pump with large clearances, because of machining tolerances or wear, exhibits a proportionally large slip. For a specific pump, the retrograde flow varies directly as the pressure differential across the pump and inversely as the fluid viscosity.
Precision displacement of thin liquids with volumes and flows measured in microliters and microliters per minute is increasingly important in laboratory and industrial procedures and instruments. Most conventional gear pumps formed with rigid materials, including plastics, incorporate clearances measured in thousandths of an inch between closely proximate elements in relative motion. Thus, they are most useful for viscous lubricating liquids such as oil and are generally operated at rotational speeds above 500 rpm. As shaft speed or pump displacement is decreased to meet microflow requirements, slip becomes a larger proportion of total pump capacity. Precise pump-controlled flow of thin liquids at low rotor speed is a difficult technical problem solved by this invention.
The term "rubber" usually denotes thermoset molded vulcanized material. "Elastomer" is a more general term for elastic materials including injection molded thermoplastic materials such as SANTOPRENE (a trademark of Monsanto), an elastomeric alloy.
The invention resides in a novel sealing arrangement comprising a compressively meshed pair of elastomeric gears (an axially self-balancing shafted driver and a shaftless driven gear which is self-balancing with respect to all forces acting on it) each of which rotates in compressive contact with the rigid interior surface of a gear pump chamber partitioned by the contacting gears such that the entire gear pump device comprises a dynamic seal between fluid input and output ports and thus provides precisely metered flow linearly proportional to rotational speed of the drive shaft down to zero revolutions per minute.
The prior art fails, however, to teach the rotary gear pump of the present invention designed for low-speed, low-flow capabilities. Those capabilities result from the provision of interference fits between elastic and rigid components of the rotary pump assembly.